A drum brake for automobiles is a component of the braking system which generates the braking force. Currently, a brake used in a typical automobile generates a brake moment by the friction between the fixed component and the rotating component, i.e., a friction-type brake. Brakes which expand by using a hydraulic cylinder are divided into drum brakes and disc brakes.
The rotating component of the frictional pair of a drum brake is the brake drum. The operating surface of the brake drum is an inner cylinder surface. The affixed component is a brake shoe. The brake shoe expands when it is driven by a braking cylinder which is controlled by a hydraulic mechanism. The different structures and operating characteristics of these drum brakes expanded by braking cylinders can be further categorized into the following types: leading-trailing shoe brakes, double-leading shoe brakes, double-trailing shoe brakes, double acting leading shoe brakes, and self-servo brakes.
FIG. 1 shows a leading-trailing shoe brake, comprising a brake drum 100, two symmetry brake shoes 200 and a double-piston braking cylinder 300. Here, the outer surface of the brake shoes is riveted with friction pads. The end of each brake shoe is supported by an anchor pin 400. When the brake is applied, the two brake shoes are driven by the hydraulic cylinder of a double-piston braking cylinder. The brake shoes expand and turn outwards on the anchor pin which is the pivot point and press on the rotating brake drums. A frictional moment (i.e., a brake moment) is created between the brake shoes and the brake drum. The direction of friction moment is opposite to the direction of rotation of the wheels, thereby creating a braking effect. To remove the braking effect, the hydraulic pressure of the hydraulic braking cylinder is released, and the brake shoes return to their original positions under the force of the return spring.
As the automobile is moving, the direction of the rotation of the brake drum is as shown by the arrow in FIG. 1. When the brake is applied, the two brake shoes turn to expand outwards on each anchor pin as the pivot point. The turning direction of the first brake shoe is opposite to the direction of rotation of the brake drum thus it is called the leading shoe. The turning direction of the second shoe is identical to the direction of rotation of the brake drum thus it is called the trailing shoe. When the automobile is backing up, the first brake shoe becomes the trailing shoe and the second brake shoe becomes the leading shoe. Since there is always a leading shoe and a trailing shoe when the brake is applied whether it is moving forward or backward, this type of brakes is called leading-trailing shoe brake.
FIG. 2 shows a double-leading shoe brake, comprising a brake drum 102, two brake shoes 202, and two single-piston braking cylinders 302. The end of each brake shoe is supported by an anchor pin 402. The brake is approximately identical to the leading-trailing shoe brake as shown in FIG. 1. The difference lies in that the brake utilizes two single-piston braking cylinders 302 respectively driving two brake shoes 202 to turn and to expand outwards. As the automobile moves forward, both of the brake shoes are leading shoes thus it is called double-leading shoe brake. This type of brakes is effective when the brake is applied as it is moving forward. However, when the automobile backs up, both of the brake shoes become trailing shoes thus the effectiveness is greatly diminished.
FIG. 3 shows a double-trailing shoe brake, comprising a brake drum 104, two brake shoes 204, and two braking cylinders 304. The end of each brake shoe is supported by an anchor pin 404. Compared to the brake of FIG. 2, the double-trailing show brake of FIG. 3 is created by turning the double-leading shoe brake 180 degrees. Apparently, the braking effect of the double-trailing shoe brake is not as desirable as that of the double-leading shoe brake when the automobile is moving forward. However, the double-trailing shoe brake is less sensitive to the change in the friction factor thus it has good stability in its braking effect.
FIG. 4 shows a double acting leading shoe brake. Compared to the leading-trailing shoe brake of FIG. 1, it has an additional braking cylinder opposite to the first braking cylinder. Both ends of both brakes shoes 206 have floating grippers. When the automobile is moving forward, the pistons of both cylinders 306 move outwards under the hydraulic force, thereby pressing the brake shoes against the rotating brake drum 106. Thereafter, under the force of the friction of the brake drum, the two brake shoes turn by a certain degree around the center of the wheel along the direction of the rotation of the brake drum. Thus the pedestal at the outside of the pistons of the cylinders is pushed back until the top of the pedestal reaches the transverse plane of the cylinder. When the automobile backs up, the brake drum exerts a friction moment in the opposition direction on the brake shoes. Thus the adjustable pedestal becomes the anchoring point of the brake shoes, and both of the brake shoes are still leading shoes. Since both of the brake shoes are leading shoes whether the automobile is moving forward or back ward, this type of brake is called double acting leading shoe brake.
FIG. 5 shows a single-self-servo brake. It utilizes a single-piston driving cylinder 308 to drive the first brake shoe 207. The end of a second brake shoe 208 is supported by an anchor pin 408. The brake shoe is connected to a second brake shoe 208 through an adjustable pole 508. When the brake is applied, the first brake shoe 207 turns to expand outwards under the force of the driving cylinder 308, pressing against the brake drum 108. Meanwhile, through the adjustable pole 508, the first brake shoe 207 affects the second brake shoe 208, making the second brake shoe 208 to also turn to expand outwards and press against the brake drum 108, creating a self-serving force.
FIG. 6 shoes a double-self-servo brake, comprising a brake drum 110, two brake shoes 210, and a double-piston driving cylinder 310. It utilizes the double-piston driving cylinder 310. The way it operates resembles that of the single-self-servo brake of FIG. 5.
The above is a brief description of the drum brakes for automobiles. The detailed structure of various prior art brakes can be found in various publications on automobile structures, e.g., “Automobile Structure” by Guan Wen-Da (Editor-in-Chief), Tsinghua University Press, September, 2004 (1st Edition).
The above mentioned drum brakes are widely used in various types of automobiles because of their desirable characteristics such as a compact and simple structure, good radiating properties, and easy to be used as parking brakes. Among the drum brakes, the double acting leading shoe brake has the best braking effectiveness since both of the brake shoes are leading shoes when the brake is applied whether it is moving forward or backward. In addition, the brake shoes of a double acting leading shoe brake are worn evenly. Thus the double acting leading shoe brakes are most widely adopted.
However, as the traveling speed of the automobiles generally increases, there is a higher demand on the effectiveness of the drum brakes. In real applications, the effectiveness of various brakes is not very satisfactory. The problems of excessively long braking time and braking distance still remain. Thus, there is a desire to have novel drum brake apparatuses that can provide better braking distance and braking time.